Arrangement at a press tool for breaking the piston/piston rod of a gas spring

ABSTRACT

The invention relates to an arrangement in a pressing tool for braking the return movement of pistons/piston rods (D 1 ) of gas-filled springs (D) and thereby also of a sheet metal holder (B) supported thereon after pressing has been performed. The invention is characterised in that each gas-filled spring device (D) is designed with a separate brake cylinder part (E) and brake piston part (F), in which the brake cylinder pail (E) is situated adjoining the outer end of the gas-filled spring (D) and in which the brake piston part (F) is situated adjoining the inner end of the piston rod (D 1 ).

[0001] The present invention relates to an arrangement in a pressingtool, more specifically in a pressing tool in which the forming processis performed using a sheet metal holder supported by a gas-filledspring, which holds the sheet metal throughout the forming process.

[0002] In sheet-metal forming operations, use is made of tools fitted ina press. The upper part of the tool follows the movement of the pressslide throughout, while other parts, such as the said sheet metalholder, for example, are at rest during a part of the press cycle. Thesprings which support the sheet metal holder are loaded or tensioned inthe pressing process and retain the sheet metal fabrication securelyduring forming, to be relieved and expanded during the upward movementof the press slide and upper part of the press after sheet metal forminghas been carried out. There is a risk at this point of the sheet metalholder lifting off from the gas-filled springs and of the finished sheetmetal fabrication coming loose uncontrollably from the sheet metalholder if the negative g-force (the retardation) becomes too greatduring the braking adjacent to the limit positions of the gas-filledsprings. If the springs are braked too rapidly after return, the holderwill not come to a stop when the springs reach their limit positions,but risks travelling on to be braked by the g-force, whereupon it fallsback down on to the springs. This “jump” by the holder (blank holderbounce) causes wear and generates noise. There is also a risk of theholder itself jumping out of its guides.

[0003] An example of the known technology in this area is given inEP-A-1074759.

[0004] The main object of the present invention is therefore to providean arrangement in a pressing tool that eliminates the risk of the holderlifting off from the gas-filled springs.

[0005] Another object of the invention is to provide an arrangementwhich incorporates a damping/braking of the piston/piston rod movementof the gas-filled spring and hence also of the movement of the holderadjacent to the extended limit position, by using existing components ofthe press.

[0006] According to the invention the aforementioned objects areachieved by an arrangement having the characteristics specified in theclaims.

[0007] By incorporating a limit position damper into the gas-filledsprings forming part of the main springs in the press, that is to saysprings with a capacity of almost 10 tonnes per spring in large presses,it is easily possible to control the retardation both of the springs andof the holder, so as to achieve gentle braking both of the holder partand then also of the finished pressed part once pressing has beencarried out.

[0008] The invention will now be described in connection with an exampleof an embodiment shown in drawings attached, in which:

[0009]FIGS. 1 and 1b show diagrams of a part of a known pressing toolwith sheet metal holder and sheet metal fabrication before andimmediately after the forming process,

[0010]FIGS. 2a-c show diagrammatic cross-sections through a modifiedgas-filled spring arrangement according to the invention permitting abraking/limit-position damping and at various degrees of extension,

[0011]FIG. 3 shows a comparative diagram of the speed profiles for thepiston rod of a gas-filled spring during a press cycle—with and withoutthe present invention, and

[0012]FIG. 4 shows a likewise comparative diagram of the movement of theholder during a press cycle—with and without the present invention.

[0013]FIG. 1s shows diagrams of a part of a known pressing tool A withsheet metal holder B and sheet metal fabrication C immediately prior tothe forming process, while FIG. 1b shows the holder part B of the sameknown pressing tool isolated from the underlying gas-filled springpiston D on completion of the press cycle.

[0014]FIG. 2a shows a first cross-sectional view through a modifiedgas-filled spring arrangement D according to the invention with brakecylinder part E and brake piston part F permitting a limit-positionbraking/damping on attainment of the extended position—here shown in itsfully nested position, however—while FIG. 2b shows the gas-filled springarrangement D with its piston/piston rod D1 in a position in which abraking has been initiated by uniting of the brake cylinder part E andthe brake piston part F and the formation of a sealed chamber G. FIG. 2cshows the gas-filled spring arrangement D with piston/piston rod D1 in afully nested limit position, in which the braking action ceases. Inorder to be able to control the braking of the piston/piston rod, thebrake cylinder part E is sealed by an external sealing ring O adjacentto the inner end of the piston/piston rod D1. A bore (not shown) betweenthe brake/damper chamber formed and the rest of the inside of the gascylinder is designed so that a desired braking characteristic isobtained for the piston/piston rod D1.

[0015]FIG. 3 shows a comparative diagram of the speed profiles for thepiston/piston rod of a gas-filled spring during a press cycle—by adashed line in the case of an arrangement with the present invention andby a solid line in that of a conventional arrangement without built-inlimiting of the speed of the piston/piston rod speed at the outer limitposition. The dashed area in FIG. 3 is a measurement of the damping ofthe piston rod movement that is obtained.

[0016]FIG. 4 shows the comparative diagram for the movement of theholder during a press cycle—with and without an arrangement according tothe present invention, the solid and the dashed line respectivelyshowing the difference between the controlled (damped) braking of theholder and the uncontrolled holder braking. The latter results in theholder coining loose from the pistons of the gas-filled springs.

[0017] The invention is not limited to the example of an embodimentdescribed above, but lends itself to modifications without departingfrom the scope of the claims specified below.

1. Arrangement in a pressing tool for braking a piston/piston rod (D1)of a gas-filled spring (D) and thereby also the return movement of asheet metal holder (B) supported thereon after pressing has beenperformed, characterised in that the gas-filled spring (D) is designedwith a separate brake cylinder part (E) and brake piston part (F), inwhich the brake cylinder part (E) is situated adjoining the outer end ofthe gas-filled spring (D) and in which the brake piston part (F) issituated adjoining the inner end of the piston rod (D1).
 2. Arrangementaccording to claim 1, characterised in that a seal (O) is arrangedbetween brake piston part (F) and brake cylinder part (E). 3.Arrangement according to claim 1 or 2, characterised in that a bore isaccommodated between the annular chamber (G), formed by brake pistonpart (F) and brake cylinder part (E), and the rest of the internalcylindrical space (B) of the gas-filled spring (D).